Escalating trends in global warming place vulnerable agricultural populations at increased risk for heat-related illness (HRI). Agricultural workers are highly susceptible to heat stress, given routine occupational exposure to hot, humid environments. The average annual heat-related death rate for agricultural workers is nearly 20 times greater than that of the U.S. civilian workforce. There is a paucity of research characterizing the hazardous work environment for this vulnerable agricultural population and no studies have documented the factors that lead to the increased heat-related morbidity and mortality observed among farmworkers. To date investigations on HRI in this population have been limited to surveys of perceptions of heat exposure, and self- reported histories of heat related symptoms and work practices to decrease risk of heat exposure. We propose to study the threat of HRI in farmworker populations built upon established research partnerships with farmworker advocacy groups in Florida. Our preliminary work has given us important information on the feasibility of this work in a vulnerable working population. In this application we propose to more thoroughly assess the nature of heat related symptoms in multiple farmworker populations by conducting sophisticated biomonitoring data to explore the relationship between individual physiologic responses to heat stress and prevalence of HRI symptoms within and between occupational settings. To achieve this goal, we will: (1) Characterize worker exposure to heat related hazards and work activity characteristics of fernery, crop, and nursery workers in Central Florida; (2) Characterize the physiologic heat stress response (e.g., HRI and PSI) and worker vulnerability to heat exposures among fernery, crop, and nursery workers in Central Florida; and (3) Determine the extent to which heat related hazards and worker vulnerability influence heat stress response among fernery, crop, and nursery workers in Central Florida. This work will provide critical, physiologically- based information regarding the heat-related hazards of agricultural work and will inform the development of interventions to decrease the risks of HRI associated with the work environment of these particularly vulnerable and unique farmworker populations. This project will support NIOSH research priorities by reducing environmental and occupational health disparities and promoting health through heat-prevention strategies in this and other at-risk occupational populations.